ircutils: split ! and @ in hostmasks from the right

This fixes /names parsing when ! is a prefix character and userhost-in-names is enabled: previously, strings such as "!@user!ident@some.host" were incorrectly split into "" for nick and "@user!ident@some.host" for ident@host.

src/ircutils.py

@@ -90,8 +90,8 @@ def splitHostmask(hostmask):
     """hostmask => (nick, user, host)
     Returns the nick, user, host of a user hostmask."""
     assert isUserHostmask(hostmask)
-    nick, rest = hostmask.split('!', 1)
-    user, host = rest.split('@', 1)
+    nick, rest = hostmask.rsplit('!', 1)
+    user, host = rest.rsplit('@', 1)
     return (minisix.intern(nick), minisix.intern(user), minisix.intern(host))
 
 def joinHostmask(nick, ident, host):

test/test_ircutils.py

@@ -266,6 +266,13 @@ class FunctionsTestCase(SupyTestCase):
     def testNickFromHostmask(self):
         self.assertEqual(ircutils.nickFromHostmask('nick!user@host.domain.tld'),
                          'nick')
+        # Hostmasks with user prefixes are sent via userhost-in-names. We need to
+        # properly handle the case where ! is a prefix and not grab '' as the nick
+        # instead.
+        self.assertEqual(ircutils.nickFromHostmask('@nick!user@some.other.host'),
+                         '@nick')
+        self.assertEqual(ircutils.nickFromHostmask('!@nick!user@some.other.host'),
+                         '!@nick')
 
     def testToLower(self):
         self.assertEqual('jemfinch', ircutils.toLower('jemfinch'))